1. Field of the Invention
The present invention relates to a light reflective structure using an internal reflection system and a method for producing the same, and a reflective display and a transflective display using the same, in particular a reflective liquid crystal display and a transflective liquid crystal display using the same.
2. Discussion of Background
There have been now widely used a reflective display and a transflective display. These displays are suited to a display screen for portable instruments since, when external light can be utilized, image can be watched without backlight, therefore reducing total power consumption. As the display element for these displays, a liquid crystal display element has been mostly used to contribute to decrease power consumption.
One of the important constituent elements that dominate the function of these reflective liquid crystal display and transflective liquid crystal display is a reflection surface. In particular, it is necessary to realize a superior reflection surface in order to effectively utilize a small amount of external light for image and to obtain desired image quality.
From this viewpoint, various structures have been used as a reflection layer for forming the reflection surface. For example, there is a method, which uses a metallic reflection surface made of an aluminum sheet. It has been known that the reflection surface in a display element is provided with a property of reflecting light in a particular direction (hereinbelow, referred as to the reflectiveness in a specific direction) and directivity in the scattering state of reflection light (hereinbelow, referred as to the scattering directivity) to obtain bright image.
There have been proposed several methods, wherein a light reflective structure exhibiting the scattering directivity in a liquid crystal cell is photolithographically produced. For example, there has been proposed a method for forming a micro slant reflector, which has been disclosed by C. J. Wen et al. (ERSO/ITRI) in SID2000 (e.g., JP-A-2000-105370). In this method, a photosensitive resin is coated on a substrate, and the coated resin is obliquely exposed through a photomask, which includes a pattern having light shielding portions and transmission portions. This method creates problems in that production costs are high because of exposure from a slant direction, and that when a proximity mask aligner is used, it is difficult to fabricate many light reflective structures in the same shape on a large scale substrate since the intensity and the distribution of light vary from location to location by the presence of the collimation angle.
There has been also disclosed a method, wherein the line widths in a photomask are continuously changed to create a transitional change in exposure values (e.g., JP-A-2000-321410). In order to continuously change the line widths in a photomask so as to create a transitional change in exposure values, it is necessary for exposure light to have a wide range of diffusivity with respect to a line spacing. This method causes problems, e.g., in that the asymmetry of an exposure distribution is decreased, and that it is necessary to make the film thickness of a photosensitive resin thicker at the initial stage in order to form a desired slant shape since the rate of changes in the exposure distribution becomes small. It is extremely difficult to uniformly and thickly coat a photosensitive resin on a glass substrate. When a proximity mask aligner adaptable to a large-scale glass substrate is used, there is no practical solution because of a limitation to the film thickness of a photosensitive resin even if the exposure has wide diffusivity. This is because commercially available photomasks suited for that sort of mask aligner have a line spacing of not shorter than 1 μm.
Although there has been recently proposed a gray scale photomask with light shielding portions having different transmissions as a method for creating the transitional change in exposure values, the gray scale photomask is not suited to a photomask for exposing a large-scale glass substrate.
There has been also disclosed a technique, wherein by using two photomasks having different patterns, a photolithographic process is carried out in two steps to form an asymmetric shape, and the asymmetric shape is made gentle by a reflow step (e.g., JP-A-2000-180610). This technique causes problems, e.g., in that it is necessary to carry out the photolithographic process in two steps, and that the provision of two photomasks raises the cost.
There has been also disclosed a technique, wherein posts having different heights are formed, the heights of the posts are changed by a melting step, and a resin is coated on the melted posts to obtain an asymmetric shape (e.g., JP-A-2001-141915). This technique causes a problem that the cost is raised since it is necessary to carry out an additional coating step to provide the resin.
With regard to a shape exhibiting both of the reflectiveness in a specific direction and the scattering directivity, there has been proposed a shape with a diffused uneven portion put on slant surfaces as disclosed by C. J. Wen et al. (ERSO/ITRI) in SID2000. Although it is supposed that this method is highly effective, there are problems in that an exposure step needs to be carried out in two steps, and that the structure is thick.
There have been also disclosed techniques, which utilize, e.g., a form obtained by asymmetrically cutting spherical bodies having an elliptic cross-section (e.g. JP-A-2001-141915, JP-A-2000-180610 and JP-A-2000-105370). However, the form thus obtained causes a problem that the usability of light is low since the form has a property of scattering light in all directions, not a property of scattering light in a limited area in the vicinity in a direction, i.e., a property of having the reflectiveness in a specific direction.
In order to obtain a property of having great light intensity and bright reflection in image using the internal reflection system, what is important is that it is possible to avoid glare due to reflection on an outer surface of the screen of a display at the time of watching the screen (hereinbelow, referred as to the glare avoiding effect) by reflecting light in a specific direction on an internal surface in order to effectively utilize surrounding light (the reflectiveness in a specific direction), and that it is easy for a user to direct her or his eyes toward the specific direction at the time of watching the screen (high visibility) by providing the scattering state of internally reflected light with directivity (the scattering directivity).